Solar cell, solar cell module, and production method for solar cell

ABSTRACT

An n-type low-doped region and a first main-surface side highly doped region, which has an n-type dopant concentration higher than that in the n-type low-doped region, are provided in an n-type crystalline silicon substrate. The first main-surface side highly doped region is arranged between the n-type low-doped region and a p-type amorphous silicon layer.

CROSS REFERENCE TO RELATED APPLICATION

The entire disclosures of Japanese Patent Application No. 2015-195306filed on Sep. 30, 2015, and No. 2016-057960 filed on Mar. 23, 2016,including specification, claims, drawings and abstract are incorporatedherein by reference in their entireties.

TECHNICAL FIELD

The disclosure relates to a solar cell, a solar cell module andproduction method for a solar cell.

BACKGROUND ART

As a conventional solar cell module, a solar cell module having a bypassdiode is known, as described in Patent Literature 1. The solar cellmodule has a plurality of solar cell strings connected in series and aplurality of bypass diodes connected in series. The solar cell stringhas a plurality of solar cells connected in series. Two solar cellstrings connected in series constitute one unit. Each of the bypassdiodes is connected in parallel to the respective units that aredifferent from each other (hereinafter referred to as a string unit).

When a solar cell(s) within a solar cell string is shaded by a barrierand the shaded area increases, electric current flows to a bypass diodeconnected in parallel to the string unit containing the solar cell(s).Likewise, a string unit containing a shaded solar cell(s) is bypassed toprevent no output from the solar cell module.

CITATION LIST Patent Literature Patent Literature 1: Japanese PatentLaid-Open Publication No. 2013-157457

With the solar cell module disclosed in Patent Literature 1, if solarcells are shaded, the string unit containing the solar cells is bypassedand thus not all the solar cells in the string unit contribute to powergeneration. Accordingly, power generation of solar cells not shadedwithin the string unit is also prevented, leading to a great decrease inpower generation performance.

SUMMARY

An object of the disclosure is to provide a solar cell that successfullysuppresses loss of power generation performance of a solar cell modulewhen shaded and a solar cell module containing the solar cell.

A solar cell according to an embodiment of the disclosure has a firstconductive-type silicon substrate, and a second conductive-typeamorphous silicon layer positioned on a first main-surface side of thesilicon substrate. The silicon substrate has a low-doped region whichhas been doped to be a first conductive-type, and a first main-surfaceside highly doped region provided between the low-doped region and thesecond conductive-type amorphous silicon layer and having aconcentration of a first conductive-type dopant higher than that in thelow-doped region.

Note that the requirement “a second conductive-type amorphous siliconlayer positioned on a first main-surface side of the silicon substrate”is satisfied in the case where the second conductive-type amorphoussilicon layer is in contact with the first main-surface side of thesilicon substrate. The requirement “a second conductive-type amorphoussilicon layer positioned on a first main-surface side of the siliconsubstrate” is also satisfied in the case where the secondconductive-type amorphous silicon layer faces the first main-surface ofthe silicon substrate with a layer such as an intrinsic semiconductorlayer sandwiched between the second conductive-type amorphous siliconlayer and the first main-surface of the silicon substrate.

According to the solar cell disclosed as an embodiment of thedisclosure, it is possible to suppress loss of power generationperformance of a solar cell module when shaded.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present disclosure will be described based on thefollowing figures, wherein:

FIG. 1 is a schematic configuration diagram showing the main part of asolar cell module according to a first embodiment;

FIG. 2 is a schematic sectional view showing a solar cell of the solarcell module;

FIG. 3 is a graph showing the results of a test example forvoltage-current characteristic of a solar cell:

FIG. 4 is a schematic sectional view showing a partial structure of asolar cell according to a reference example:

FIG. 5 is a schematic sectional view showing the solar cell of the firstembodiment comparable to FIG. 4;

FIG. 6 is a schematic configuration diagram showing the main part of thesolar cell module according to a reference example;

FIG. 7 is a schematic sectional view showing the solar cell main part ofa second embodiment;

FIG. 8 is a schematic sectional view showing the solar cell main part ofa third embodiment;

FIG. 9 is a schematic sectional view showing the solar cell main partaccording to a modified example of the third embodiment;

FIG. 10 is a schematic sectional view showing a solar cell of a fourthembodiment;

FIG. 11 is a schematic sectional view showing a solar cell according toa modified example of the fourth embodiment; and

FIG. 12 shows a modified example of a method for forming a highly dopedregion according to an embodiment of the present application.

FIG. 13 is a schematic sectional view showing the main part of the solarcell module of the first embodiment.

DESCRIPTION OF EMBODIMENTS

Now, embodiments according to the present disclosure (hereinafterreferred to as the embodiments) will be more specifically describedbelow with reference to the accompanying drawings. The specific shapes,materials, numerical values, directions and others set forth in thedescription are just examples for facilitating understanding of thisdisclosure and can be appropriately varied depending upon e.g., usage,purpose and specification. Furthermore, although a plurality ofembodiments and modified examples are contained below, using thecharacteristic parts of them appropriately in combination falls withinthe scope assumed. The drawings used in the embodiments as a referenceare schematically illustrated and the dimensional ratios of componentsshown in the drawings are sometimes different from actual dimensions. Inthe specification, the description: “substantially . . . ” means, if itis illustrated by taking the description “substantially the entireregion” as an example, that not only the entire region but also theregion that is recognized as substantially the entire region isincluded.

First Embodiment

FIG. 1 is a schematic configuration diagram showing the main part of asolar cell module 50 of the first embodiment.

As shown in FIG. 1, the solar cell module 50 has 6 solar cell strings 20and 3 bypass diodes 30. The 6 solar cell strings 20 are connected inseries. The solar cell string 20 has 12 solar cells 10, which areconnected in series by a wiring material 16. The 3 bypass diodes 30 areconnected in series. A single unit 22 (hereinafter referred to as astring unit) is constituted of 2 solar cell strings connected in series.Each of the bypass diodes 30 is connected in parallel to one of themutually different string units 22.

Note that the flow indicated by dashed line A in FIG. 1 is the flow ofelectric current produced by power generation of the solar cell 10. Ahatched single solar cell 10 k in FIG. 1 is a solar cell shaded by abarrier (not shown). The power generation mechanism of a solar cellmodule 50 having a shaded solar cell 10 k will be described later withreference to e.g., FIG. 3.

FIG. 2 is a schematic sectional view showing a solar cell 10. As shownin FIG. 2, the solar cell 10 has an n-type (first conductive-type)crystalline silicon substrate (hereinafter referred to as the n-typesilicon substrate) 1, a first i-type amorphous silicon layer 2, a p-type(second conductive-type) amorphous silicon layer (hereinafter referredto as the p-type amorphous silicon layer) 3, a second i-type amorphoussilicon layer 4 and an n-type amorphous silicon layer 5. The n-typesilicon substrate 1 may be an n-type polycrystalline silicon substrateand preferably an n-type monocrystal silicon substrate.

The first i-type amorphous silicon layer 2 is formed on a firstmain-surface of the n-type silicon substrate 1. The p-type amorphoussilicon layer 3 is provided on a first main-surface side of the n-typesilicon substrate 1. In other words, the p-type amorphous silicon layer3 is provided on the first i-type amorphous silicon layer 2 on theopposite side to the side facing the n-type silicon substrate 1. Thesecond i-type amorphous silicon layer 4 is provided on the secondmain-surface of the n-type silicon substrate 1. The n-type amorphoussilicon layer 5 is provided on the second i-type amorphous silicon layer4 on the opposite side to the side facing the n-type silicon substrate1. Note that, a small amount of oxygen atom may be contained in theinterface between the n-type silicon substrate 1 and the amorphoussilicon layer or the neighborhood thereof.

The first i-type amorphous silicon layer 2, p-type amorphous siliconlayer 3, second i-type amorphous silicon layer 4 and n-type amorphoussilicon layer 5 have a function of suppressing recombination ofphotogenerated carriers. These silicon layers 2, 3, 4 and 5 are suitablyformed by a chemical vapor deposition (CVD) method, in particular, aplasma CVD method. As the source gas for use in film-formation of thesilicon layers 2, 3, 4 and 5, a silicon-containing gas such as SiH₄,Si₂H₆ or a gas mixture of the silicon-containing gas and H₂ is suitablyused. As a dopant gas for forming the p-type or n-type amorphous siliconlayer 3 and 5, for example, B₂H₆ or PH₃ is suitably used. The additionamount of impurity such as P and B may be small and a gas mixturecontaining SiH₄ and H₂ can be used.

The first and second i-type amorphous silicon layers 2 and 4 are eachpreferably an i-type hydrogenated amorphous silicon layer (i-typea-Si:H). The p-type amorphous silicon layer 3 is preferably a p-typehydrogenated amorphous silicon layer (p-type a-Si:H). The n-typeamorphous silicon layer 5 is preferably an n-type hydrogenated amorphoussilicon layer (n-type a-Si:H). The layer (i-type a-Si:H) can be formedby the CVD method using a source gas prepared by diluting SiH₄ with H₂.The layer (p-type a-Si:H) is formed by using a source gas prepared byadding B₂H₆ to SiH₄ and diluting the gas mixture with hydrogen. Thelayer (n-type a-Si:H) is formed by using a source gas containing PH₃ inplace of B₂H₆. Note that each of the amorphous silicon layers 2 to 5 isnot necessarily hydrogenated. The film formation method for each of thesemiconductor layers is not particularly limited. The first and secondi-type amorphous silicon layers 2, 4 may contain a small amount ofdopant for the reasons of manufacturing process.

As shown in FIG. 2, the n-type silicon substrate 1 has a low-dopedregion 11 and a first main-surface side highly doped region 12. Thelow-doped region 11 is doped to be an n-type. The first main-surfaceside highly doped region 12 is provided between the low-doped region 11and the ptype amorphous silicon layer 3 and has an n-type dopantconcentration higher than that in the low-doped region 11. The n-typedopant concentrations in the low-doped region 11 and the firstmain-surface side highly doped region 12 will be described later withreference to FIG. 3. The first main-surface side highly doped region 12is provided over the entire surface of the low-doped region 11 on theside of the p-type amorphous silicon layer 3. The first main-surfaceside highly doped region 12 is an n⁺ region doped with a larger amountof an n-type dopant than that in the low-doped region 11. Each of thelow-doped region 11 and the first main-surface side highly doped region12 is formed by e.g., an ion implantation method, a thermal diffusionmethod, a plasma doping method or an epitaxial growth method. As then-type dopant, e.g., P, As and Sb are used, and in particular, P issuitably used. P is suitably doped while suppressing generation ofdefects by using a gas mixture with POCl₃ gas and applying a heattreatment. If an ion implantation method is employed, high temperatureannealing and RTA (Rapid Thermal Annealing) are preferably used incombination in order to reduce defects to be produced by ionimplantation.

If a thermal diffusion method or a plasma doping method is used forforming the first main-surface side highly doped region 12, aconcentration gradient is formed, which is a phenomenon where the(dopant) concentration gradually increases with distance from thelow-doped region 11 of the n-type silicon substrate 1. If an epitaxialgrowth method is used, compared to the case where, for example, athermal diffusion method is used, a dopant concentration can be rapidlyincreased at the boundary position between the low-doped region 11 andthe first main-surface side highly doped region 12 and the dopantconcentration in the entire first main-surface side highly doped region12 can be easily equalized.

The solar cell 10 is assumed to receive light from the side of then-type amorphous silicon layer 5. As shown in FIG. 2, the solar cell 10has a transparent conductive layer 6 and a rear-side collector electrode7 which are sequentially provided in this order on the p-type amorphoussilicon layer 3, on the rear-side (opposite to the light receivingside). The solar cell 10 has a transparent conductive layer 8 and afront-side collector electrode 9, which are sequentially provided inthis order on the n-type amorphous silicon layer 5 on the front-side(the light receiving side). The transparent conductive layer 6 is formedover substantially the entire region of the rear surface of the p-typeamorphous silicon layer 3, whereas the transparent conductive layer 8 isformed over substantially the entire region of the front-side surface ofthe n-type amorphous silicon layer 5. Each of the transparent conductivelayers 6 and 8 has transparency and conductivity. Each of thetransparent conductive layers 6 and 8 is constituted of, for example, ametal oxide such as In₂O₃, ZnO, SnO₂ or TiO₂. These metal oxides may bedoped with a dopant such as Sn, Zn, W, Sb, Ti, Ce, Zr, Mo, Al, and Ga.

The rear-side and front-side collector electrodes 7 and 9 are formed byscreen printing of a conductive paste with a pattern having for example,a large number of finger parts and bus bar parts lower in number thanthe finger parts. The rear-side collector electrode 7 is preferablyformed so as to have a larger area than the front-side collectorelectrode 9 and the number of the finger parts of the rear-sidecollector electrode 7 is preferably larger than that of the front-sidecollector electrode 9. Note that the structure of the electrodes is notparticularly limited. For example, the rear-side collector electrode maybe constituted of a metal layer, which is formed over substantially theentire region of the transparent conductive layer.

The solar cell 10 is assumed to receive light from the side of then-type amorphous silicon layer 5. However, the solar cell may receivelight from the side of the p-type amorphous silicon layer.Alternatively, the solar cell may receive light from both sides, i.e.,the side of the p-type amorphous silicon layer and the side of then-type amorphous silicon layer. The first main-surface side of then-type silicon substrate 1 is preferably specified as a non-lightincident surface (a rear surface). This is because carrier mobility islow in a highly doped region. Accordingly, if the highly doped region isspecified as a light incident surface (a light receiving surface),carriers generated in the highly doped region are easily recombined,with the result that the short circuit photocurrent density decreases inconnection with the intensity of the light to be irradiated. Thedecrease in the short circuit photocurrent density can be suppressed bysetting the highly doped region in the opposite side to the lightincident surface. Herein, the light incident surface of the solar cell10 refers to the main surface upon which light is mainly incident.Beyond 50% to 100% of light incident upon the solar cell 10 enters fromthe light incident surface. The rear surface refers to the surfaceopposite to the light incident surface, in short, a main surface formedon the transparent conductive layer and being larger in collectorelectrode area.

FIG. 13 is a sectional view showing the solar cell module 50 constitutedof the solar cells 10 shown in FIG. 2. In the solar cell module 50, sunlight mainly enters in the direction pointed by arrow S. The solar cellmodule 50 has a plurality of the solar cells 10, a front-side protectingmaterial 51 provided on the light incident surface-side of the solarcells 10 and a rear-side protecting material 54 provided on therear-side of the solar cells 10. The solar cell module 50 further has asurface-side seal material 52 packed in the space between the front-sideprotecting material 51 and the solar cells 10, a rear-side seal material53 placed in the space between the rear-side protecting material 54 andthe solar cells 10. The solar cells 10 are sandwiched with therespective protecting materials such that the front-side collectorelectrodes 9 face the light incident surface side of the solar cellmodule 50 and the rear-side collector electrodes 7 face the rear side ofthe solar cell module 50.

The front-side protecting material 51 is a plate formed of a materialhaving transparency, such as glass or a resin material, sufficientlytransmitting light within a wavelength range contributing to powergeneration by the solar cell 10. The rear-side protecting material 54 isa plate or sheet made of glass or a resin material, which does notnecessarily have transparency. As the surface-side seal material 52 andrear-side seal material 53, a seal material, such as polyolefin and anethylene-vinyl acetate copolymer, usually used in a solar cell modulemay be used. The rear-side seal material 53 may contain e.g., a titaniumoxide (TiO₂) particle and a zinc oxide (ZnO₂) particle and may bepigmented with e.g., white. In the solar cell module 50 as mentionedabove, the solar cells 10 are arranged such that the first main-surfaceof the n-type silicon substrate 1, on which the first main-surface sidehighly doped region 12 is to be formed, faces the rear side of the solarcell module 50.

FIG. 3 is a graph showing the voltage-current characteristic of thesolar cell 10 according to a test example. In the test example, theaverage doping concentration of P doped in the low-doped region 11 isabout 1.8×10¹⁵ cm⁻³. In FIG. 3, solid line f shows the voltage-currentcharacteristic in the case where the average concentration of P doped inthe first main-surface side highly doped region 12 is 1×10¹⁸ cm⁻³.Dotted line g shows the voltage-current characteristic in the case wherethe average concentration of P doped in the first main-surface sidehighly doped region 12 is 5×10¹⁸ cm⁻³. Dot-and-dash line r shows thevoltage-current characteristic in the case where the averageconcentration of P doped in the first main-surface side highly dopedregion 12 is 1×10¹⁹ cm⁻³. Solid line h shows the voltage-currentcharacteristic of the solar cell according to a reference example wherean n-type silicon substrate is constituted of a low-doped region alonewithout the first main-surface side highly doped region.

As indicated by solid line h, in the case of the solar cell according tothe reference example where the first main-surface side highly dopedregion was not provided, even if a voltage drop was increased up to 15V, no electric current flowed. In contrast, in the case of the solarcell indicated by solid line f, if a voltage drop was about 2 V or more,electric current flow gradually started and the electric current valuefinally reached a plateau at around 3 A. In the case of the solar cellindicated by dotted line g and dot-and-dash line r where the averageconcentration of P is 5×10¹⁸ cm⁻³ or more, when the voltage drop reached1.5 V or more, electric current flow rapidly started, and finally, thevoltage drop reached nearly 2V or less at an electric current of 6 A.Likewise, satisfactory voltage-current characteristic was obtained.

FIG. 4 is a schematic sectional view showing a partial structure of asolar cell 110 according to the reference example. The solar cell 110has an n-type crystalline silicon substrate 101 having an average dopingconcentration P of 1×10¹⁴ to 1×10¹⁶ cm⁻³. The solar cell 110 has astructure obtained by sequentially laminating an i-type amorphoussilicon layer 111 and a p-type amorphous silicon layer 112 in this orderon the n-type crystalline silicon substrate 101. FIG. 5 is a schematicsectional view showing the solar cell 10 of the embodiment, comparableto FIG. 4. The solar cell 10 has the n-type silicon substrate 1 havingthe low-doped region 11 and the first main-surface side highly dopedregion 12. The solar cell 10 has a structure obtained by subsequentlylaminating the i-type amorphous silicon layer 11 and the p-typeamorphous silicon layer 12 in this order on the n-type silicon substrate1. The solar cell 110 of the reference example differs from the solarcell 10 in that a low-doped region and a first main-surface side highlydoped region are not provided in the n-type crystalline siliconsubstrate 101 and that the n-type crystalline silicon substrate 101 hasa uniform n-type dopant concentration.

It has been confirmed that the power generation performance of the solarcell 10 is the same as that of the solar cell 110 of the referenceexample. Note that it has also been confirmed that the power generationperformance of a solar cell according to a modified example of theembodiment and the power generation performance of the solar cells ofthe following embodiments and modified examples thereof are the same asthe power generation performance of the solar cell 110 of the referenceexample.

FIG. 6 is a schematic configuration diagram of a main part of a solarcell module 150 according to the reference example. The solar cellmodule 150 differs from the solar cell module 50 of the embodiment onlyin that the solar cell 110 shown in FIG. 4 is used in place of the solarcell 10. Now, the power generation performance of the solar cell module50 of the embodiment shown in FIG. 1 when the solar cells 10 are shadedwill be described in comparison with the power generation performance ofthe solar cell module 150 of the reference example having a shaded solarcell 110. In the solar cell module 150 of the reference example, astring unit 122 is constituted of two solar cell strings 120 connectedin series. Each bypass diode 130 is connected in parallel to one of themutually different string units 122.

In the reference example shown in FIG. 6, a single solar cell 110 k isassumed to be shaded in the solar cell module 150. In this case, it isrequired to have an excessive voltage drop in order for electric currentto flow to the solar cell 110 k, as shown by reference symbol h (thereference example) in FIG. 3. Thus, in this case, as shown in FIG. 6,electric current flows to a bypass diode 130 connected in parallel tothe string unit 122 and the string unit 122 having the solar cell 110 kis bypassed. Accordingly, electric current flows through the passageindicated by the dotted line pointed by arrow D in FIG. 6. Twenty foursolar cells 110 present in the hatched region within the string unit 220do not contribute to power generation. Likewise, a significant outputloss is produced.

In contrast, in the case where a single solar cell 10 k is assumed to beshaded in the solar cell module 50 of the embodiment shown in FIG. 1,electric current rapidly flows in the solar cell 10 k due to a smallvoltage drop as indicated by dotted line g in FIG. 3. Accordingly, sincethe electric current flowing through the solar cell module 50 isgenerally not so large, the power consumption at the solar cell 10 k,which is expressed by a product of the aforementioned small voltage andthe “not-so-large” electric current, can be suppressed to a small value.In consideration of the fact that a large number of the solar cells 10not shaded in the string unit 22 other than the solar cell 10 kparticipate in power generation, the output loss of the solar cellmodule 50 of the embodiment is greatly reduced, compared to the solarcell module 150 of the reference example. Note that in the case of theembodiment, a single bypass diode 30 is connected in parallel to eachstring unit 22. This is because if a plurality of the solar cells 10present in the same string unit 22 are shaded, output loss can besomewhat reduced by bypassing the string unit 22.

According to the first embodiment, the n-type low-doped region 11 andthe n-type first main-surface side highly doped region 12 which has adopant concentration higher than the n-type low-doped region 11 areprovided in the n-type silicon substrate 1. The n-type firstmain-surface side highly doped region 12 is provided between the n-typelow-doped region 11 and the p-type amorphous silicon layer 3. Electriccurrent can be supplied to the shaded solar cell 10 k at a low voltagedrop. Thus, loss of the power generation performance of the solar cellmodule 50, produced when the solar cell 10 is shaded, can be suppressed.

Note that in the first embodiment, a case where a layer for suppressingrecombination (hereinafter referred to as the recombination suppressinglayer) is formed by sequentially laminating the first i-type amorphoussilicon layer 2 and the p-type amorphous silicon layer 3 in this orderon the first main-surface side of the n-type silicon substrate 1 hasbeen illustrated. In addition, it has also been illustrated that thefirst i-type amorphous silicon layer is preferably an i-typehydrogenated amorphous silicon layer (i-type a-Si:H) and that the p-typeamorphous silicon layer 3 is preferably a p-type hydrogenated amorphoussilicon layer (p-type a-Si:H). However, on the first main-surface sideof the n-type silicon substrate 1, a recombination suppressing layerother than these layers may be formed. On the first main-surface side ofthe n-type silicon substrate 1, a recombination suppressing layer formedof a material selected from the following (1) to (6) and including theselayers can be suitably formed: (1) p-type a-Si:H, (2) p-type a-SiC:H,(3) a laminate of i-type or p-type a-Si:H and high-concentration p-typea-Si:H (laminate of i-type or p-type a-Si:H/high concentration p-typea-Si:H), (4) a laminate of i-type or p-type a-Si:H/high concentrationp-type hydrogenated microcrystalline silicon (p-type pc-Si:H) (5) alaminate of i-type or p-type a-SiC:H/high concentration p-type a-Si:H,and (6) a laminate of i-type or p-type a-SiC:H/high concentration p-typepc-Si:H. Furthermore, a recombination suppressing layer containing ap-type layer other than these, for example, a recombination suppressinglayer including a non-hydrogenated p-type layer, can be formed. Herein,the term “high-concentration” means, if it is illustrated by taking “alaminate of p-type a-Si:H/high concentration p-type a-Si:H”, as anexample, that the dopant concentration of the latter layer is higherthan that in the former layer. More specifically, this expression meansthat this is a structure obtained by laminating two layers havingdifferent dopant amounts.

Note that a case where the second i-type amorphous silicon layer 4 andthe n-type amorphous silicon layer 5 are sequentially laminated in thisorder on the second main-surface side of the n-type silicon substrate 1to form a recombination suppressing layer has been explained. It hasbeen also illustrated that the second i-type amorphous silicon layer 4is preferably an i-type hydrogenated amorphous silicon layer (i-typea-Si:H) and that the n-type amorphous silicon layer 5 is preferably ann-type hydrogenated amorphous silicon layer (n-type a-Si:H). However, arecombination suppressing layer other than these layers may be formed onthe second main-surface of the n-type silicon substrate 1. Arecombination suppressing layer formed of a material selected from thefollowing (7) to (12) and including these layers can be suitably formed:(7) n-type a-Si:H, (8) n-type a-SiC:H, (9) a laminate of i-type orn-type a-Si:H and high concentration n-type a-Si:H (laminate of i-typeor n-type a-Si:H/high concentration n-type a-Si:H), (10) a laminate ofi-type or n-type a-Si:H/high concentration n-type hydrogenatedmicrocrystalline silicon (n-type pc-Si:H), (11) a laminate of i-type orn-type a-SiC:H/high concentration n-type a-Si:H, (12) a laminate ofi-type or n-type a-SiC:H/high concentration n-type pc-Si:H on the secondmain-surface sides of the n-type silicon substrate 1. Furthermore, arecombination suppressing layer containing a n-type layer other thanthese, for example, a recombination suppressing layer including anon-hydrogenated n-type layer, can be formed. Herein, the term“high-concentration” means, if it is illustrated by taking “a laminateof n-type a-Si:H/high concentration n-type a-Si:H”, as an example, thatthe dopant concentration of the latter layer is higher than that in theformer layer. More specifically, this expression means that this is astructure obtained by laminating two layers different having dopantamounts.

Note that a case where a protecting layer is not formed on therecombination suppressing layer present on the first and secondmain-surface sides of the n-type silicon substrate 1 has beenillustrated. However, a protecting layer may be formed on at least oneside of the recombination suppressing layers on the first and secondmain-surface sides of the n-type silicon substrate 1. The protectinglayer has a function of, for example, suppressing damage of therecombination suppressing layer, thereby suppressing reflection oflight. The protecting layer is preferably constituted of a materialhaving high transparency and suitably constituted of e.g., silicon oxide(SiO₂), silicon nitride (SiN) or silicon oxynitride (SiON).

On the surface of the n-type silicon substrate 1, a texture structure(not shown) may be provided. The texture structure refers to a structurehaving an uneven surface for increasing light absorption of the n-typesilicon substrate 1 by suppressing surface reflection, and is formed,for example, on the light receiving surface alone or on both the lightreceiving surface and the rear surface. The texture structure can beformed by anisotropic etching the (100) plane of the monocrystal siliconsubstrate with an alkaline solution. In this manner, a pyramidal unevenstructure (the plane (111) as an oblique plane) is formed in the surfaceof the monocrystal silicon substrate. The distance between the highestpart and the lowest part of uneven portions of the texture structure is,for example, 1 μm to 15 μm.

Needless to say, the thickness of each of the layers 1 to 5, 11 and 12can be appropriately varied depending upon the specification. Forexample, the thickness of the n-type silicon substrate 1 can be set at50 μm to 300 μm. The thickness of the n-type first main-surface sidehighly doped region 12 can be set at, for example, 200 nm or less, maybe within the range of several to 500 nm, and suitably 15 nm to 200 nm,more preferably 50 nm to 100 nm. The thickness of each of therecombination suppressing layers present on first and secondmain-surface sides of the n-type silicon substrate 1 can be set at 1 nmto 50 nm and preferably 2 nm to 15 nm.

If the surface doping concentration of the low-doped region 11 is 1×10¹⁴to 1×10¹⁶ cm⁻³. then when the surface doping P concentration of thefirst main-surface side highly doped region 12 is 1×10¹⁸ cm⁻³ or more,suitable results are obtained. An example of a suitable range of thesurface doping P concentration of the first main-surface side highlydoped region 12 is from 2×10¹⁸ cm⁻³ to 2.5×10¹⁸ cm⁻³. As describedabove, the first and second i-type amorphous silicon layers 2, 4 maycontain a small amount of a dopant. In this case, the surface doping Pconcentration of the first main-surface side highly doped region 12preferably falls within a range, which is present on the lowerconcentration side of the above suitable concentration range, in short,e.g., 0.5×10¹⁸ cm⁻³ to 2.0×10¹⁸ cm⁻³. However, the surface dopingconcentrations in the low-doped region and the first main-surface sidehighly doped region are not limited to these values. This is because theeffect of the invention of the present application can be obtained byforming the first main-surface side highly doped region 12 having asurface doping concentration larger than the low-doped region 11. Evenif the average doping concentration of the first main-surface sidehighly doped region 12 is smaller than 1×10¹⁸ cm⁻³ if the surface dopingconcentration of the low-doped region 11 is, for example, 1×10¹⁵ cm⁻³ ormore and more preferably 5×10¹⁵ cm⁻³ or more, electric current flows dueto a small voltage drop of about 2 V. Thus, even in this case, comparedto the reference example where electric current does not flow until avoltage drop reaches 15 V, as indicated by a reference symbol h in FIG.3, a tunnel effect is extremely easily produced. Accordingly, in thiscase, output loss of a solar cell module produced when a solar cell isshaded can be suppressed depending on the specification. At this time,the surface doping P concentration in the first main-surface side highlydoped region 12 is further preferably 1×10¹⁹ cm⁻³ or more and 5×10¹⁹cm⁻³ or less.

A case where the first main-surface side highly doped region 12 isprovided over the entire surface of the low-doped region 11 on thep-type amorphous silicon layer 3 side has been illustrated. However, thefirst main-surface side highly doped region may be provided on a part ofthe surface of the low-doped region on the p-type amorphous siliconlayer side. For example, the first main-surface side highly doped regionmay be provided only on both ends or the center of the low-doped region11 in the direction substantially perpendicular to the thicknessdirection.

A case where the first conductive-type is n-type and the secondconductive-type is p-type has been illustrated, but a case where thefirst conductive-type is p-type and the second conductive-type is n-typemay be accepted.

A case where the solar cell module 50 has 6 solar cell strings 20 andthe solar cell string 20 has 12 solar cells 10 has been illustrated.However, the number of solar cell strings contained in the solar cellmodule may be other than 6, and the number of solar cells contained ineach solar cell string may be other than 12. Furthermore, a case where asingle bypass diode 30 is connected in parallel to a string unit 22consisting of 2 solar cell strings 20 connected in series, has beenillustrated, but a single bypass diode may be connected in parallel to astring unit consisting of solar cell strings, the number of which isother than 2 (including one), connected in series. The solar cell modulemay not have a bypass diode.

Second Embodiment

FIG. 7 is a schematic sectional view showing the main part of a solarcell 210 according to a second embodiment. In FIG. 7, a transparentconductive layer and a collector electrode are not shown. In the solarcell of the second embodiment, the same reference numerals are used todesignate the same structural elements corresponding to those in thefirst embodiment and any further explanation is omitted. In the solarcell of the second embodiment, any further explanation on theoperational effect and modified example in common with those in thefirst embodiment is omitted and only the structure, operational effectand modified example, which differ from those of the first embodiment,will be described.

The second embodiment differs from the first embodiment in that ann-type crystalline silicon substrate 201 has a second main-surface sidehighly doped region 213 in addition to the low-doped region 11 and thefirst main-surface side highly doped region 12.

The second main-surface side highly doped region 213 is provided on asecond main-surface side of the n-type crystalline silicon substrate201. The second main-surface side highly doped region 213 is providedbetween the low-doped region 11 and the n-type amorphous silicon layer5. The second main-surface side highly doped region 213 is provided overthe entire surface of the low-doped region 11 on the side of the n-typeamorphous silicon layer 5.

The second main-surface side highly doped region 213 has an averagen-type dopant concentration higher than that in the low-doped region 11.The average n-type doping concentration of the second main-surface sidehighly doped region 213 may be the same as or different from the averagen-type doping concentration of the first main-surface side highly dopedregion 12. The average n-type doping concentration of the secondmain-surface side highly doped region 213 can be appropriately variedand is preferably 1×10¹⁸ cm⁻³ or more, for example, 2×10¹⁸ cm⁻³ to2.5×10¹⁸ cm⁻³. As described above, the first and second i-type amorphoussilicon layers 2, 4 may contain a small amount of dopant. In this case,the average n-type doping concentration of the second main-surface sidehighly doped region 213 preferably falls within a range which is presenton the lower concentration side of the above suitable concentrationrange, in short, e.g., 0.5×10¹⁸ cm⁻³ to 2.0×10¹⁸ cm⁻³.

The layer thickness of the second main-surface side highly doped region213 may be the same as or different from that of the first main-surfaceside highly doped region 12. The layer thickness of the secondmain-surface side highly doped region 213 can be appropriately varieddepending upon the specification. The layer thickness of the secondmain-surface side highly doped region 213 can be set at, for example,200 nm or less, may be within the range of several to 500 nm, andsuitably 15 nm to 200 nm, more preferably 50 nm to 100 nm. If the n-typehighly doped region is provided in the n-type crystalline siliconsubstrate 201 on the side of the n-type amorphous silicon layer 5(second main-surface side), recombination of photogenerated carriers issuppressed and output is improved.

According to the second embodiment, since the second main-surface sidehighly doped region 213, which has a surface dopant concentration higherthan that in the low-doped region 11, is provided between the low-dopedregion 11 and the n-type amorphous silicon layer 5, recombination ofphotogenerated carriers can be suppressed and output is improved.Particularly preferably, the n-type surface doping concentration of thefirst main-surface side highly doped region 12 is set at 1×10¹⁸ cm⁻³ ormore and the n-type surface doping concentration of the secondmain-surface side highly doped region 213 is set at 1×10¹⁷ cm⁻³ more.This is because an effect of suppressing a decrease in output of thesolar cell module 50 when a solar cell (like 10 k) is shaded and aneffect of suppressing recombination of photogenerated carriers are bothremarkably exerted. Note that at this time, the n-type surface dopingconcentration of the first main-surface side highly doped region 12 isfurther preferably 1×10¹⁹ cm⁻³ or more and 5×10¹⁹ cm⁻³ or less.

In the second embodiment, a case where the second main-surface sidehighly doped region 213 is provided over the entire surface of thelow-doped region 11 on the side of the n-type amorphous silicon layer 5,has been illustrated. However, the second main-surface side highly dopedregion may be provided only on a part of the surface of the low-dopedregion on the side of the n-type amorphous silicon layer. For example,the second main-surface side highly doped region may be provided only atboth ends or the center of the low-doped region 11 in the directionsubstantially perpendicular to the thickness direction, or alternativelymay be provided in plane like dots.

Third Embodiment

FIG. 8 is a schematic sectional view showing the main part of a solarcell 310 according to a third embodiment. A transparent conductive layerand a collector electrode are not shown in FIG. 8. In the solar cell 310of the third embodiment, the same reference numerals are used todesignate the same structural elements of a solar cell 210 correspondingto those in the second embodiment and further explanation is omitted. Inthe solar cell 310 of the third embodiment, any further explanation onthe operational effect and modified example in common with those of thesolar cells 10 and 210 in the first and second embodiments is omittedand only the structure, operational effect and modified example, whichdiffer from those of the solar cells 10 and 210 in the first and secondembodiments, will be described.

The third embodiment is the same as the second embodiment in that ann-type crystalline silicon substrate 301 has the n-type low-doped region11, the first main-surface side highly doped region 12 and the secondmain-surface side highly doped region 213. In contrast, the thirdembodiment is different from the second embodiment in that the n-typecrystalline silicon substrate 301 has first and second highly dopedside-regions 314 a and 314 b, which are provided so as to cover bothside surfaces of the n-type low-doped region 11.

As shown in FIG. 8, the n-type crystalline silicon substrate 301 has thefirst highly doped side-region 314 a at one of the sides of the n-typelow-doped region 11 in the direction substantially perpendicular to thethickness direction. The n-type crystalline silicon substrate 301 hasthe second highly doped side-region 314 b at the other side of then-type low-doped region 11 in the direction substantially perpendicularto the thickness direction. Each of the first and second highly dopedside-regions 314 a and 314 b extends in the thickness direction of then-type crystalline silicon substrate 301. The first highly dopedside-region 314 a connects the first main-surface side highly dopedregion 12 and the second main-surface side highly doped region 213 atone of the sides in the aforementioned substantially perpendiculardirection. The second highly doped side-region 314 b connects the firstmain-surface side highly doped region 12 and the second main-surfaceside highly doped region 213 at the other side in the aforementionedsubstantially perpendicular direction.

Each of the first and second highly doped side-regions 314 a and 314 bhas an average n-type dopant concentration higher than that in thelow-doped region 11. The average n-type dopant concentration of thefirst highly doped side-region 314 a may be the same as the averagen-type dopant concentration of one or more regions of the firstmain-surface side highly doped region 12 and the second main-surfaceside highly doped region 213. The average n-type dopant concentration ofthe first highly doped side-region 314 a may differ from the averagen-type dopant concentrations of both regions 12 and 213. The averagen-type dopant concentration of the second highly doped side-region 314 bmay be the same as the average n-type dopant concentration of one ormore regions of the first main-surface side highly doped region 12, thesecond main-surface side highly doped region 213 and the first highlydoped side-region 314 a. The average n-type dopant concentration of thesecond highly doped side-region 314 b may differ from the average n-typedopant concentrations of these all regions 12, 213 and 314 a. Each ofthe first and second highly doped side-regions 314 a and 314 bpreferably has an average n-type dopant concentration of 1×10¹⁸ cm⁻³ ormore, for example 2×10¹⁸ cm⁻³ to 2.5×10¹⁸ cm⁻³. The layer thickness ofeach of the first and second highly doped side-regions 314 a and 314 bin the direction perpendicular to the substrate thickness direction canbe appropriately varied depending upon the specification. The layerthickness of each of the first and second highly doped side-regions 314a and 314 b in the direction perpendicular to the substrate thicknessdirection can be set at, for example, 200 nm or less, may be within therange of several to 500 nm and suitably 15 nm to 200 nm, more preferably50 nm to 100 nm.

According to the third embodiment, since both side surfaces of then-type crystalline silicon substrate 301 are covered with the first andsecond highly doped side-regions 314 a and 314 b, surface recombinationat both side surfaces of the n-type crystalline silicon substrate 301can be reduced and power generation performance can be improved.

In addition, since the n-type highly doped region is provided so as tosurround the low-doped region 11 of the n-type crystalline siliconsubstrate 301, each of the highly doped regions 12, 213, 314 a and 314 bcan be simultaneously and easily formed around the low-doped region 11by e.g., heat diffusion using POCl₃ gas. Accordingly, the number ofsteps for manufacturing the solar cell 310 can be reduced and cycle timecan be shortened. In this case, highly doped regions 12, 213, 314 a and314 b all have the same average n-type dopant concentration ofpreferably 1×10¹⁸ cm⁻³ or more, and further preferably 1×10¹⁹ cm⁻³ ormore and 5×10¹⁹ cm⁻³ or less.

Note that in the third embodiment, a case where the first and secondhighly doped side-regions 314 a and 314 b are provided so as to coverthe both side surfaces of the n-type low-doped region 11 has beenillustrated. However, the highly doped side-region may be provided onlyon one of the side surfaces of the n-type low-doped region 11.

Another case where the first and second highly doped side-regions 314 aand 314 b connect the first main-surface side highly doped region 12 andthe second main-surface side highly doped region 213, has beenillustrated. However, at least one highly doped side-region may notconnect the first main-surface side highly doped region and the secondmain-surface side highly doped region. At least one highly dopedside-region is provided only on a part of the side surfaces of thelow-doped region. If the second main-surface side highly doped region isnot present, the highly doped side-region may be provided on at least apart of the side surfaces of the low-doped region. As shown in FIG. 9,more specifically, the schematic sectional view showing the main part ofa solar cell 410 according to a modified example of the thirdembodiment, the n-type highly doped region may not be provided in thecenter of an n-type silicon substrate 401 on the first main-surfaceside. As shown in FIG. 9, the first main-surface side highly dopedregion may be provided only at both ends of the n-type silicon substrate401 in the direction substantially perpendicular to the thicknessdirection. Note that a transparent conductive layer and a collectorelectrode are not shown in FIG. 9.

Fourth Embodiment

FIG. 10 is a schematic sectional view showing a solar cell 510 of afourth embodiment. In the solar cell 510 of the fourth embodiment, anyfurther explanation on the operational effect and modified example incommon with the solar cell 10 in the first embodiment is omitted andonly the structure, operational effect and modified example, whichdiffer from the solar cell 10 of the first embodiment, will bedescribed.

The solar cell 510 of the fourth embodiment differs from those of thefirst to third embodiments in that a p-type semiconductor layer 550 andan n-type semiconductor layer 560 are provided on the first main-surfaceside of an n-type crystalline silicon substrate 501.

As shown in FIG. 10, the solar cell 510 has the n-type crystallinesilicon substrate (hereinafter referred to simply as the substrate) 501,the p-type semiconductor layer 550 and the n-type semiconductor layer560, which are formed on the first main-surface of the substrate 501.The p-type semiconductor layer 550 and the n-type semiconductor layer560 have a portion mutually overlapped in the thickness direction. Aninsulating layer 570 is provided in the space of the overlapped portionin the thickness direction. The light receiving surface of the substrate501 corresponds to the surface opposite to the first main-surface of thesubstrate 501. The solar cell 510 has an n-type low-doped region 511 andan n-type first main-surface side highly doped region 512. The n-typefirst main-surface side highly doped region 512 has an average n-typedoping concentration higher than that in the n-type low-doped region511. The n-type first main-surface side highly doped region 512 isformed over the entire surface of the n-type low-doped region 511 on theopposite side to the light receiving surface side.

A part of the n-type first main-surface side highly doped region 512 isprovided between the n-type low-doped region 511 and the p-typesemiconductor layer 550. The p-type semiconductor layer 550 isconstituted of, for example, a laminated structure formed of an i-typeamorphous silicon layer and a p-type amorphous silicon layer, asdescribed in the first embodiment. The p-type semiconductor layer 550may be constituted of a recombination suppressing layer formed of amaterial selected from the above (1) to (6).

The other part of the n-type first main-surface side highly doped region512 is provided between the n-type low-doped region 511 and the n-typesemiconductor layer 560. The n-type semiconductor layer 560 isconstituted of a laminated structure formed of for example, an i-typeamorphous silicon layer and an n-type amorphous silicon layer, asdescribed in the first embodiment. The n-type semiconductor layer 560may be constituted of a recombination suppressing layer formed of amaterial preferably selected from the above (7) to (12).

On the side of the p-type semiconductor layer 550 opposite to the sideof the substrate 501, a p-side electrode 580 is provided. The p-sideelectrode 580 consists of a transparent conductive layer formed on thep-type semiconductor layer 550 and a collector electrode formed on thetransparent conductive layer. On the side of the n-type semiconductorlayer 560 opposite to the side of the substrate 501, an n-side electrode590 is provided. The n-side electrode 590 consists of a transparentconductive layer formed on the n-type semiconductor layer 560 and acollector electrode formed on the transparent conductive layer. A shadowloss is suppressed by arranging both the p-side electrode 580 and then-side electrode 590 on the substrate 501 on the opposite side to thelight receiving surface side.

As a material for each of the transparent conductive layers and amanufacturing method thereof, the material and manufacturing method asdescribed in the first embodiment can be suitably used. Each collectorelectrode may be formed by using a conductive paste and preferablyformed by electrolytic plating. The aforementioned rear-side andfront-side collector electrode is each formed of a metal such as Ni, Cuand Ag, may be a laminated structure of an Ni layer and a Cu layer, andmay have a tin (SN) layer on the outermost surface in order to improvecorrosion resistance. As a suitable laminated structure of thetransparent conductive layer and the collector electrode there is alaminated structure of a transparent conductive layer formed of indiumtin oxide (ITO) and a collector electrode formed of Cu.

The p-side electrode 580 and the n-side electrode 590 are not in contactwith each other and are electrically separated. The solar cell 510 has apair of electrodes formed only on the rear side of the n-typecrystalline silicon substrate 501. Holes produced in the powergeneration region are collected by the p-side electrode, whereaselectrons are collected by the n-side electrode.

The p-type semiconductor layer 550 and the n-type semiconductor layer560 are both laminated on the rear surface of the n-type crystallinesilicon substrate 501 to form a p-type region and an n-type region onthe rear surface. The p-type region and the n-type region arealternately arranged, for example, in a single direction, and engagedwith each other like a comb tooth (shape pattern) in the planar view. Inthe example shown in FIG. 10, a part of the p-type semiconductor layer550 overrides a part of the n-type semiconductor layer 560 to formindividual semiconductor layers (p-type region, n-type region) withoutspace between them on the rear surface of the n-type crystalline siliconsubstrate 501. At the portion at which the p-type semiconductor layer550 overlaps with the n-type semiconductor layer 560, the insulatinglayer 570 is provided. The insulating layer 570 is formed of, forexample, silicon oxide, silicon nitride or silicon oxynitride. In eachlayer and each region, the dopant concentration, layer thickness andproduction method for each layer can be appropriately varied dependingupon the specification. As the concentration, layer thickness andproduction method for each layer, those described in the firstembodiment and the modified example can be employed.

According to the fourth embodiment, a part of the first main-surfaceside highly doped region 512, which is formed over the entire surface ofthe low-doped region 511 on the opposite side to the light receivingsurface side, is positioned between the n-type low-doped region 511 andthe p-type semiconductor layer 550. On the other hand, the other part ofthe first main-surface side highly doped region 512 is positionedbetween the n-type low-doped region 511 and the n-type semiconductorlayer 560. Thus, the layers corresponding to first and secondmain-surface side highly doped regions 12 and 213 in the secondembodiment can be simultaneously formed only by providing one highlydoped region 512 on the low-doped region 511 on the opposite side to thelight receiving surface side. Accordingly, an effect of suppressing adecrease in output of the solar cell module when a solar cell is shadedand an effect of suppressing recombination of photogenerated carrierscan be easily obtained.

In the fourth embodiment, a case where the first main-surface sidehighly doped region 512 is provided over the entire surface of then-type low-doped region 511, has been illustrated. However, the firstmain-surface side highly doped region may not be provided over theentire surface of the n-type low-doped region. FIG. 1I is a schematicsectional view showing a solar cell 610 according to a modified exampleof the fourth embodiment. In the modified example, the same referencenumerals are used to designate the same structural elementscorresponding to those in the fourth embodiment and any furtherexplanation is omitted. As shown in FIG. 11, in the modified example, afirst main-surface side highly doped region 612 is provided partlybetween an n-type low-doped region 611 and the p-type semiconductorlayer 550 in an n-type crystalline silicon substrate 601. Like themodified example, if the first main-surface side highly doped region 612is not provided between the n-type low-doped region 611 and the n-typesemiconductor layer 560, a decrease in output of the solar cell modulewhen a solar cell 610 is shaded, can be suppressed. Note that the firstmain-surface side highly doped region is provided over the entire regionbetween the n-type low-doped region of the n-type crystalline siliconsubstrate and the p-type semiconductor layer, and is not providedbetween the n-type low-doped region of the n-type crystalline siliconsubstrate and the n-type semiconductor layer. Alternatively, the firstmain-surface side highly doped region is provided over the entire regionbetween the n-type low-doped region and the p-type semiconductor layerin the n-type crystalline silicon substrate and may be provided partlybetween the n-type low-doped region and the n-type semiconductor layerin the n-type crystalline silicon substrate. Alternatively, the firstmain-surface side highly doped region is provided only partly betweenthe n-type low-doped region and the p-type semiconductor layer in then-type crystalline silicon substrate and may be provided only partlybetween the n-type low-doped region and the n-type semiconductor layerin the n-type crystalline silicon substrate.

Modified Example

In each of the aforementioned embodiments, a highly doped region wasformed by e.g., an ion implantation method, a thermal diffusion method,a plasma doping method or an epitaxial growth method. A highly dopedregion with a predetermined conductive-type dopant may be formed by amethod belonging to the thermal diffusion method, i.e., a method ofdiffusing the predetermined conductive-type dopant in a crystallinesilicon substrate by applying a step of bringing a solution containingthe predetermined conductive-type dopant into contact with the surfaceof the crystalline silicon substrate, followed by a heat treatment stepof the crystalline silicon substrate, or alternatively, by preparing asolution containing the predetermined conductive-type dopant foroxidizing silicon, bringing the solution into contact with thecrystalline silicon substrate to form a silicon oxide layer containingthe predetermined conductive-type dopant on the surface of thecrystalline silicon substrate, and thereafter applying the heattreatment step to diffuse the predetermined conductive-type dopant fromthe silicon oxide layer to the surface of the crystalline siliconsubstrate. For example, an n-type highly doped region may be formed byusing a solution containing an n-type dopant, such as P, As and Sb. Acase of using a particularly preferably n-type dopant P will be morespecifically described by way of a modified example of the thirdembodiment.

FIG. 12 is a view illustrating a method of forming highly doped regions12, 213, 314 a and 314 b, according to the modified example. Note thatthe same reference numerals are used to designate the same or similarstructural elements corresponding to those in the third embodiment.

First, a chemical solution for forming a silicon oxide containing P isprepared, which serves to form a silicon oxide layer 370 on the surfaceof the n-type crystalline silicon substrate 301 having a surface texturestructure. The n-type crystalline silicon substrate 301 consists of thelow-doped region 11 having an n-type dopant concentration lower thanthose of the highly doped regions 12, 213, 314 a and 314 b. Then, then-type crystalline silicon substrate 301 having a texture structure on asurface is soaked in the chemical solution for forming a silicon oxideto form a P-containing oxide layer 370 having a thickness of several toabout 20 angstroms, over the entire surface of the n-type crystallinesilicon substrate 301 (S1). Then, the n-type crystalline siliconsubstrate 301 having the P-containing oxide layer 370 on the surface issubjected to a heat treatment in a nitrogen atmosphere or an oxygenatmosphere (S2). P contained in the oxide layer 370 diffuses from thesurface of the n-type crystalline silicon substrate 301 toward theinterior by this heat treatment and forms highly doped regions 12, 213,314 a and 314 b, which are n⁺ regions containing P in a higherconcentration than other regions, in the interface between the n-typecrystalline silicon substrate 301 and the oxide layer 370 and theneighborhood thereof. Thereafter, the n-type crystalline siliconsubstrate 301 is soaked in a hydrofluoric acid (HF) solution to removethe oxide layer 370 to expose the highly doped regions 12, 213, 314 aand 314 b in the surface of the n-type crystalline silicon substrate 301(S3).

The chemical solution for forming a silicon oxide used in the wetprocess step of SI is an acidic aqueous solution containing an n-typedopant and is not limited as long as it is an aqueous solution servingfor oxidizing the surface of silicon. In the modified example, asolution mixture of a nitric acid aqueous solution with a concentrationof 85 mass % and a phosphoric acid aqueous solution with a concentrationof 75 mass % is used. The mixing ratio of individual aqueous solutions,more specifically, the volume ratio of the nitric acid aqueous solution:the phosphoric acid aqueous solution, falls in the range ofsatisfactorily 10:90 to 50:50 and preferably 20:80 to 40:60. Theprocessing temperature of step S preferably falls in the range of forexample, 20° C. to 100° C. and preferably about 50° C. to 80° C.

The acidic aqueous solution is not limited to a solution containingphosphoric acid (orthophosphoric acid) and nitric acid and may contain,for example, other acids such as hydrochloric acid and a hydrogenperoxide solution in place of nitric acid or in addition to nitric acid.As phosphoric acid serving as an n-type dopant source, metaphosphoricacid may be used. The acidic aqueous solution may contain a P-containingcompound other than phosphoric acid.

In the above, a method using an acidic aqueous solution is mentioned;however, another chemical solution such as a neutral chemical solutionmay be used as long as an alkaline aqueous solution dissolving then-type crystalline silicon substrate 301 is not used.

The heat treatment (S2), which is a treatment for diffusing a dopant, iscarried out under an oxygen or nitrogen atmosphere, and particularlypreferably under an oxygen atmosphere. Alternatively, the heat treatmentmay be carried out under a water-vapor atmosphere. The temperature ofthe heat treatment falls within the range of preferably 700° C. to 1000°C. and further preferably 800° C. to 950° C. However, the temperaturecan be appropriately controlled. The time for the heat treatment is, forexample, about 10 to 60 minutes, but the time can be appropriatelycontrolled.

The P-containing oxide layer 370 formed by use of the chemical solutionfor forming a silicon oxide may cover the entire surface of the n-typecrystalline silicon substrate 301 or may be formed on a part or in theform of islands. The thickness and formation area of the oxide layer 370can be controlled by varying the mixing ratio of a plurality of acidaqueous solutions contained in the chemical solution for forming asilicon oxide for use in forming the oxide layer 370, concentrations ofthe individual acid aqueous solutions, and formation conditions such astemperature, and time for the oxide layer 370. In diffusing P in then-type crystalline silicon substrate 301, the diffusion depth and theconcentration of P can be controlled by varying e.g., the temperatureand time for the heat treatment.

In the above modified example of the third embodiment, a method forforming a highly doped region by bringing a solution containing apredetermined conductive-type dopant into contact with a substratefollowed by applying a predetermined heat treatment to diffuse thepredetermined conductive-type dopant into the substrate has beenillustrated. Highly doped regions in the first, second and fourthembodiments may be formed in the same manner as in this method. Then-type crystalline silicon substrate 301 having both a highly dopedregion and a low-doped region in the surface may be formed, for example,by forming the P-containing oxide layer 370 over the entire surface ofthe n-type crystalline silicon substrate 301, removing a part of theoxide layer 370 and applying a heat treatment. Alternatively, the n-typecrystalline silicon substrate 301 having both a highly doped region anda low-doped region in the surface may be formed by forming theP-containing oxide layer 370 over the entire surface of the n-typecrystalline silicon substrate 301, applying a heat treatment to form thehighly doped region over the entire surface of the n-type crystallinesilicon substrate 301, and removing a part of the highly doped regionthus formed by etching, etc.

The n-type crystalline silicon substrate 301 having a highly dopedregion formed by a wet process and a heat treatment as described abovemay be subjected to the same manufacturing step as in each of the aboveembodiments to get a solar cell completed. The step of manufacturing asolar cell includes a step of forming a p-type amorphous silicon layeron the first main-surface side of the n-type crystalline siliconsubstrate 301, from which the oxide layer 370 is removed. It ispreferable that the step of manufacturing a solar cell further includesa step of forming an n-type amorphous silicon layer on the secondmain-surface side of n-type crystalline silicon substrate 301, fromwhich the oxide layer 370 is removed. Even in the case of using a dopantdiffusion method employing a wet process and a heat treatment, a solarcell having satisfactory voltage-current characteristic can be formedand suitable results are obtained. Note that a method for forming ahighly doped region described in the modified example is just anexample. Other than the methods shown in the modified example, a highlydoped region can be formed by applying a solution containing a dopant tothe surface of a crystalline silicon substrate by e.g., a spin coatingmethod or a spray method and then applying a heat treatment.

REFERENCE SIGNS LIST

1, 201, 301, 401, 501, 601 n-type silicon substrate, 2 first-i typeamorphous silicon layer, 3 p-type amorphous silicon layer, 4 secondi-type amorphous silicon layer, 5 n-type amorphous silicon layer, 10,210, 310, 410, 510, 610 solar cell, 11, 511, 611 low-doped region, 12,512, 612 first main-surface side highly doped region, 16 Wiringmaterial, 20 solar cell string, 30 bypass diode, 50 solar cell module,213 second main-surface side highly doped region, 314 first highly dopedside-region, 314 b second highly doped side-region. 550 p-typesemiconductor layer, 560 n-type semiconductor layer, 370 oxide layer

1. A solar cell comprising a first conductive-type silicon substrate,and a second conductive-type amorphous silicon layer positioned on afirst main-surface side of the silicon substrate, wherein the siliconsubstrate has a low-doped region which has been doped to be a firstconductive-type, and a first main-surface side highly doped region whichis provided between the low-doped region and the second conductive-typeamorphous silicon layer and has a concentration of a firstconductive-type dopant higher than that in the low-doped region.
 2. Thesolar cell according to claim 1, comprising a first conductive-typeamorphous silicon layer positioned on a second main-surface side of thesilicon substrate, wherein the silicon substrate has the low-dopedregion and a second main-surface side highly doped region which isprovided between the low-doped region and the first conductive-typeamorphous silicon layer and has a concentration of the firstconductive-type dopant higher than that in the low-doped region.
 3. Thesolar cell according to claim 1, wherein the silicon substrate has ahighly doped side-region which is provided so as to cover the sideextending in a thickness direction of the silicon substrate and has aconcentration of the first conductive-type dopant higher than that inthe low-doped region.
 4. The solar cell according to claim 1, whereinthe first main-surface side highly doped region is provided on both endsof the silicon substrate in a direction substantially perpendicular to athickness direction.
 5. The solar cell according to claim 1, wherein asurface dopant concentration in the first main-surface side highly dopedregion is 1×10¹⁸ cm⁻³ or more.
 6. The solar cell according to claim 1,wherein the low-doped region comprises a n-type crystalline silicon, andthe first main-surface side highly doped region is an n⁺ region which isdoped with a larger amount of n-type dopant than that in the low-dopedregion.
 7. The solar cell according to claim 1, wherein the firstmain-surface of the silicon substrate is a non-light incident surface.8. The solar cell according to claim 1, wherein the silicon substrate,on the first main-surface side, further has an amorphous silicon layerof the first conductive-type, which is positioned adjacent to theamorphous silicon of the second conductive-type in a directionsubstantially perpendicular to the thickness direction of the siliconsubstrate.
 9. A solar cell module having series-connected solar cellsaccording to claim
 1. 10. The solar cell module according to claim 9,comprising a bypass diode, which is connected in parallel to two or moresolar cells connected in series and contained in the solar cells.
 11. Amethod for manufacturing a solar cell, comprising a step of forming asilicon oxide layer containing an n-type dopant on the surface of ann-type crystalline silicon substrate by soaking the substrate in anacidic aqueous solution containing the n-type dopant, a step of formingan n⁺ region, in which the concentration of the n-type dopant is higherthan in other regions, at least at the interface between the n-typecrystalline silicon substrate and the silicon oxide layer by diffusingthe n-type dopant from the silicon oxide layer to the interior of then-type crystalline silicon substrate by treating the n-type crystallinesilicon substrate with heat, a step of exposing the n⁺ region of then-type crystalline silicon substrate by removing the silicon oxidelayer, and a step of forming a p-type amorphous silicon layer on a firstmain-surface side of the n-type crystalline silicon substrate from whichthe silicon oxide layer has been removed.
 12. The method formanufacturing a solar cell according to claim 11, comprising a step offorming a second main-surface side n-type amorphous silicon layer of then-type crystalline silicon substrate from which the silicon oxide layerhas been removed.
 13. The method for manufacturing a solar cellaccording to claim 11, wherein the acidic aqueous solution containsphosphoric acid and nitric acid.